Recent advances and perspectives in analytical methodologies for monitoring the bioavailability of trace metals in environmental solid substrates

In the last decades, researchers have realised that the impact of trace elements (TE) in environmental solid substrates on ecological systems and biota cannot be ascertained appropriately by means of total metal content measurements. Assessment of TE chemical forms, types of binding and reactivity of their associations with particulate forms has, thus, been commonly performed via batch-wise equilibrium-based sequential extraction fractionation methods able to discern TE bound to different soil-phase compartments. In this paper, novel analytical strategies for monitoring the mobility, bioavailability and the eventual impact of anthropogenic TE in environmental solids are addressed. The potential of passive dosimeters based on microdialysis sampling for on-site, real-time monitoring of chemical contaminants in pore soil solution is thoroughly discussed and critically compared with active microsamplers. Recent miniaturised configurations designed for following the fate of target pollutants and the on-going chemical changes occurring at local soil sites, e.g., the rhizosphere environment, at high temporal resolution are also presented in detail. Kinetic information on the lability of the various TE forms associated to soil phases under simulated environmental changing conditions - that yield improved knowledge on short-term hazards of TE for the environment - can be obtained in a fully automated mode by means of flow-through microcolumn fractionation procedures. The current state-of-the-art of the flowing stream approach, so-called sequential injection analysis, in terms of implementation of on-line dynamic fractionation is described, and illustrated via selected examples comprising the well-accepted three-step SM&T sequential fractionation protocol.